Quartzite Xenoliths Research Articles

Multi-stage crustal accretion by magmatic flare-up and quiescence intervals in the western margin of the São Francisco Craton: U-Pb-Hf and geochemical constraints from the Almas Terrane

The global plate tectonic regime in early Paleoproterozoic times is highly debated. The interval 2.45–2.2 Ga is known for a minima in juvenile magmatism, but this is not a global phenomenon. New results of whole-rock geochemistry and U–Pb-Hf analysis in igneous and detrital zircons, allied with existing isotopic and geophysical data, allow to identify and constrain the duration of magmatic flare-up and quiescence events in the western São Francisco Paleoplate. Igneous samples yield ages indicating three accretionary magmatic events, an older with ages ca. 2476.4 ± 9 Ma to 2462 ± 13 Ma, an intermediate at 2390 ± 14 Ma, and a younger from 2235 ± 26 Ma to 2201 ± 5 Ma, all presenting magmatic arc geochemical signatures. Xenoliths of quartzite and volcanic tuff from the upper greenstone sequence (Morro do Carneiro Fm.) are hosted in the 2211 ± 9 Ma tonalite and the maximum depositional age of the Morro do Carneiro basin is dated 2234 ± 12 Ma, indicating a syn-orogenic setting for this basin. Detrital zircon UPb age distribution for quartzites of the greenstone sequence shows peaks at 2.65, 2.47, 2.39, 2.27 and 2.23 Ga. Altogether, the studied rocks record an accretionary orogeny with four distinct episodes: Episode S1: 2.52–2.46 Ga, ɛHf(t) values from +0.57 to +6.36; Episode S2: 2.43–2.37 Ga, ɛHf(t) values from +0.10 to +4.30; Episode R1: 2.32–2.26 Ga, ɛHf(t) values from +1.61 to −7.23 (from detrital zircons); Episode R2: 2.24–2.20 Ga, ɛHf(t) values from +0.39 to −2.73. These early Paleoproterozoic accretionary orogenies mark the onset of amalgamation of the São Francisco continental paleoplate that surrounds the craton, with accretions of an exotic micro-block and continental magmatic arcs, indicating evolution from dominant Siderian juvenile magmatism to Rhyacian crustal magmatism. These patterns show striking similarities to the orogenies in the Mineiro Belt and North China Craton.

Mineralogy, geochemistry and40Ar–39Ar geochronology of the Barda and Alech complexes, Saurashtra, northwestern Deccan Traps: early silicic magmas derived by flood basalt fractionation

AbstractMost continental flood basalt (CFB) provinces of the world contain silicic (granitic and rhyolitic) rocks, which are of significant petrogenetic interest. These rocks can form by advanced fractional crystallization of basaltic magmas, crustal assimilation with fractional crystallization, partial melting of hydrothermally altered basaltic lava flows or intrusions, anatexis of old basement crust, or hybridization between basaltic and crustal melts. In the Deccan Traps CFB province of India, the Barda and Alech Hills, dominated by granophyre and rhyolite, respectively, form the largest silicic complexes. We present petrographic, mineral chemical, and whole-rock geochemical (major and trace element and Sr–Nd isotopic) data on rocks of both complexes, along with40Ar–39Ar ages of 69.5–68.5 Ma on three Barda granophyres. Whereas silicic magmatism in the Deccan Traps typically postdates flood basalt eruptions, the Barda granophyre intrusions (and the Deccan basalt flows they intrude) significantly pre-date (by 3–4 My) the intense 66–65 Ma flood basalt phase forming the bulk of the province. A tholeiitic dyke cutting the Barda granophyres contains quartzite xenoliths, the first being reported from Saurashtra and probably representing Precambrian basement crust. However, geochemical–isotopic data show little involvement of ancient basement crust in the genesis of the Barda–Alech silicic rocks. We conclude that these rocks formed by advanced (70–75 %), nearly-closed system fractional crystallization of basaltic magmas in crustal magma chambers. The sheer size of each complex (tens of kilometres in diameter) indicates a very large mafic magma chamber, and a wide, pronounced, circular-shaped gravity high and magnetic anomaly mapped over these complexes is arguably the geophysical signature of this solidified magma chamber. The Barda and Alech complexes are important for understanding CFB-associated silicic magmatism, and anorogenic, intraplate silicic magmatism in general.

КОНТАМИНАЦИЯ МАГМЫ, ОСОБЕННОСТИ ПЕТРОГЕНЕЗИСА И РАСПРЕДЕЛЕНИЕ РУДНОГО ВЕЩЕСТВА В ПОРОДАХ НИКЕЛЕНОСНОЙ ФОРМАЦИИ СРЕДИННО-КАМЧАТСКОГО МАССИВА (часть третья)

This is the final article that describes various for petro- and oreogenesis consequences of the contamination of nickel-bearing magma with siliceous material (quartzites), which led to the replacement of ultrabasic ore-hosting rocks (cortlandites) with ultramafic (amphibole orthopyroxenites). Besides, the paper describes the products of metamagmatic variations of pyroxene ore cumulates in biotite-amphibole rocks inported by water-potassium fluid. The author shows the unconventional results of fluid influence on similar cumulates with relics of quartzite xenoliths accompanied by the formation of ore granophyric autobreccies, as well as ore-bearing derivatives from the process of magma mixing (auto-contamination). We introduce an intraformational classification of copper-nickel ores by their associations with host rocks of different composition and origin — from juvenile magmatic ultrabasites, contaminated and ultramafites and melanodiorites to hybrid granitoids. The dynamics of the formation of intrusive ore-magmatic systems and the plausible reason of differences in the ore content of large and small intrusions are discussed.

On the origin of hot metasedimentary quartzites in the lower crust of continental arcs

Volcanic arcs associated with subduction zones are thought to be the primary building blocks of continents. The composition of the magmas, particularly in continental arcs, is the product of mixing between differentiation of juvenile magmas and pre-existing crustal wallrock, the former being typically mafic and the latter more silicic. Because the upper continental crust is on average thought to be more silicic than the mafic lower crust, mixing with silicic endmembers should occur primarily in the upper crust. However, we show here that the lower crust of continental arcs contains silicic metasediments. We examine garnet-bearing, granulite-facies sedimentary quartzite xenoliths from the Sierra Nevada batholith in California, a Cretaceous continental arc. The quartzites have equigranular textures and contain quartz (>50%), plagioclase (<30%), garnet (10%), and small amounts (<1%) of rutile, aluminosilicate, biotite, monazite, zircon, graphite and trace orthopyroxene. Cathodoluminescent images show zircons with rounded detrital cores mantled by metamorphic overgrowths. Hf isotopic model ages and U–Pb upper intercept ages, for a given zircon, are similar, but the zircon population shows variable protolith ages ranging from Proterozoic to Archean. In contrast, all zircons share similar lower intercept U–Pb ages (103±10Ma), which coincide with the peak of arc magmatism in the Sierra Nevada. The Precambrian protolith ages are similar to North American cratonal basement, and together with the abundance of quartz and detrital zircons, suggest that these quartzites represent ancient, passive margin sediments instead of juvenile active margin sediments in the oceanic trench and accretionary prism. Importantly, these quartzites record peak metamorphic temperatures and pressures of 700–800°C using Ti-in-quartz thermometry and 0.7–1.1GPa using garnet–aluminosilicate–plagioclase thermobarometry, indicating that these xenoliths experienced significant heating and possible partial melting in the lower crust, most likely related to arc magmatism as suggested by similarities between the lower intercept U–Pb ages and the ages of plutonism in the Sierra Nevada. Possible mechanisms by which these sediments were transported into the lower crust include continental underthrusting beneath the continental arc, underplating by buoyant slab-derived sedimentary diapirs, or viscous downflow of country rock in response to diapiric ascent of plutons. Continental underthrusting has been independently documented during the Sevier orogeny, coinciding with the peak of arc magmatism. We thus speculate that supracrustal rocks may have been underthrusted into deep crustal magmatic zones. Regardless of how these metasediments arrived in the lower crust, our observations indicate that silicic metasediments occur in the lower crust of volcanic arcs, not just in the upper crust as is commonly thought. Transport of metasediments into deep crustal magmatic zones should influence the composition of arc magmas and continental crust in general.

Emplacement of a xenolith-rich sill, Lajitas, Texas

A 2–15m thick hawaiite sill in the Cretaceous Boquillas Formation west of the Big Bend National Park, Texas, locally contains a zone 1–12m thick containing 25vol% xenoliths. The inclusions, ranging in size from 2cm to 1m, are dominated by cumulate gabbros and white quartzite. The mafic inclusions broke from the sidewall cumulates in a reservoir that fed the hawaiite sill. Many xenoliths show contacts between quartzite and igneous rock, marked by clinopyroxene crystals perpendicular to the contacts. The quartzite xenoliths are permeated variably by quenched mafic liquid (now devitrified glass with plagioclase+clinopyroxene+smectite), and some quartz grains are fringed by inverted tridymite. Stratigraphic reconstruction indicates that the sill was emplaced at a depth shallower than 0.5km, but the source of the Paleozoic quartzite and sidewall cumulate xenoliths was at least 1km below the sill.The high concentration of xenoliths, their lack of imbrication, orientation, or sorting, the preservation of unbroken thin slabs of limestone, and the absence of xenoliths at the top and bottom of the sill all support emplacement by plug flow of xenolith-charged magma in the center, lubricated by less viscous xenolith-free magma at the margins. Crystal-size distribution of plagioclase indicates that mean crystal-residence time for cumulates was about 75years, and crystallization time at the aphyric upper margin of the sill was about 2years.

Vredefort bronzite granophyre: chemical evidence for origin as a meteorite impact melt

Computer-assisted compositional modelling studies demonstrate that the unusual chemical composition of the Bronzite Granophyre in the Vredefort structure, South Africa can be produced by the impact melting and mixing of four pre-Vredefort target rocks which constitute the lower part of the Vredefort stratigraphic section: Ventersdorp Lava, Witwatersrand Shale, Witwatersrand Quartzite and Old Granite. Similar models show that a simple igneous assimilation mechanism (total assimilation of the same wallrock types), using a wide range of possible parent magmas, cannot duplicate the granophyre composition unless geologically unreasonable amounts of wallrock (> 100% of the parent magma) are assimilated. The unusual granophyre composition reflects the incorporation of significant amounts of basalt (30–80%) into the impact melt from which the granophyre was derived. For various granophyre compositions, the granite component varies inversely with basalt, from about 45-5%, and shale is relatively uniform at 5–20%. A significant and nearly constant amount of quartzite (about 20%) is an unusual feature of the granophyre which, given the generally refractory character of quartzite wallrock, is difficult to explain by assimilation. The granophyre displays a significant range of chemical variation, which is similar to the range observed in impact melt bodies from large and small terrestrial impact structures. These variations apparently reflect actual differences in the relative amounts of different target rocks incorporated into discrete volumes of impact melt rather than variations in the composition of individual target rock components. These results are consistent with the view that the granophyre is an impact melt and that the Vredefort structure formed by the impact of an extraterrestrial object. This interpretation also explains several long-known geological features: (1) the close relationship between granophyre and pseudotachylite, both of which were formed simultaneously by shock waves of different intensities, (2) the absence of shock effects (pseudotachylite and shatter cones) in the granophyre, and (3) the occurrence of quartzite xenoliths in the granophyre at great distances from the nearest quartzite exposures.

Absence of shock-metamorphic effects in the Bushveld Complex, South Africa: results of an intensive search

An extensive search was made for distinctive shock-metamorphic effects in order to test the theory that the Bushveld Complex (BC) was formed by the impact of large extraterrestrial objects. Field and petrographie studies concentrated on two geological environments in which current impact cratering theories predict the formation and preservation of such shock-metamorphic effects as shatter cones, unusual fragmental or melt breccias, and shock lamellae in quartz. These are: (1) pre-BC quartzite xenoliths in the Rooiberg Group, a thick sequence of acid lavas (rhyolites and rhyodacites) and related rocks, proposed by earlier workers as a possible crater-filling impact melt; and (2) the locally observable contact between the Rooiberg Group and the underlying pre-BC basement (Pretoria Group), a contact possibly corresponding to the original crater floor in established impact structures. More than 320 samples of pre-BC quartzite were collected and examined petrographically for shock-metamorphic effects. No megascopic shock-metamorphic effects (shatter cones or unusual breccias) were observed in any outcrop. In thin section, no petrographic shock effects (e.g., shock lamellae hi quartz and kink bands in biotite) were observed in any sample. The quartzites from both the basement (80 samples) and the Rooiberg xenoliths (245 samples) are virtually undeformed. Cataclastic textures are rare, and individual quartz grains show only normal metamorphic deformation lamellae (Böhm lamellae) and single or multiple sets of fluid inclusions arranged along irregular planes which appear to be healed fractures. Although some of these parallel sets superficially resemble so-called “decorated” shock lamellae, universal stage measurements of their orientation demonstrate that they are totally different. The absence of both megascopic and microscopic shock-metamorphic effects in a large suite of carefully selected samples strongly indicates that impact models for the BC are not correct and that the BC is not an impact structure. This conclusion also agrees with: (1)the complex stratigraphy now recognized in the lower Rooiberg Group, which is not consistent with an impact melt origin, (2) the absence in the Rooiberg Group of any of the distinctive melt-rich fragmentai breccias which dominate the crater-filling deposits in established large impact structures, and (3) the undeformed character of the basement rocks below the Rooiberg Group and the apparently concordant contact between them.

Recrystallization of Zircon as an Indication of Contact Metamorphism

ZIRCON (ZrSiO4) is generally regarded as the most refractory accessory mineral present in the common rock types1. According to Poldervaart2, incipient granulation and recrystallization of zircon when exposed to ultra-metamorphic processes can be expected only in a stage as high as the sillimanite grade. Investigations in quartzite xenoliths in the southern norite belt of the Bushveld Igneous Complex of the Transvaal (South Africa), however, indicate definitely that these processes can be caused by usual contact metamorphism and that the foregoing conceptions should be revised.

Quartzite xenoliths in the Tertiary magmas of the Southern Highlands, N.S.W

Quartzite xenoliths in the Tertiary magmas of the Southern Highlands, N.S.W

Quartzite Xenoliths from the Ballachulish Granodiorite

AbstractSmall quartzite xenoliths scattered throughout the granodiorite are mantled by broad mafic coronas, the inner part of which is composed largely of augite and the outer part mainly of hornblende. The formation of the coronas is examined and some general features of magmatic reaction with quartzite inclusions are discussed.

A Xenolithic Sill at Tallabrista, Co. Donegal, Ireland

AbstractAn epidiorite sill emplaced in quartzites becomes charged with xenoliths of quartzite and of an amphibole-rich rock possibly derived from underlying limestone horizons. The matrix of the xenolith-rich part of the sill is studied with large hornblende-biotite “crystals” that may represent small limestone xenoliths that have been basified and homogenized.

Demonstrations in petrogenesis from Kiloran Bay, Colonsay

In 1911 Dr. W. B. Wright briefly described the felspathized quartzite blocks in the hornblendite of Port Easdale, Kiloran Bay, Colonsay, Hebrides. He showed that the exposures provide a conclusive demonstration that the action of hornblendite magma on quartzite xenoliths was to convert them to alkali-felspar and quartz, so that in the final stage ‘one can recognize in numerous angular and rounded patches of felspathic material, without visible xenolithic core, the ghosts of former masses of quartzite’.

Marginal and Contact Phenomena of the Dorback Granite

The granite mass of Dorback in the Braes of Abernethy, a few miles east of the village of Tomintoul, occupies an area of from 5 to 6 square miles, the plutonic rocks outcropping through the siliceous schists and granulites of the Central Highlands of Scotland. For a highland district the exposures are rather poor, since the igneous rocks only rarely appear from under a thick covering of peat and glacial drift, but towards the eastern margin of the mass the complex is well exposed in the valleys trenched by the Allt Iomadaidh and tributary streams. Here the interest of the plutonic rocks is twofold, for they form a complex of acid, intermediate, and basic types exhibiting considerable variation in petrography, and they include a series of xenoliths of quartzite, schist, and limestone which range from a few inches in diameter to a great mass of limestone 1½ miles in length. This xenolith has been mapped by the Geological Survey of Scotland, and it was the occurrence of a contact between limestone and plutonic rocks at this locality that led to the investigations of the authors.

The Braefoot Outer Sill, Fife - Part 1

I. Introduction. The Braefoot Outer Sill is the higher of the two thick sills intrusive in the Lower Oil Shale Group, whose horse-shoe-shaped outcrops can be traced more or less continuously from Dalgety Bay to Aberdour. Early references to the sill are to be found in Hay Cunningham’s Geology of the Lothians (1838, p. 127),1 and in Charles Maclaren’s Geology of Fife and the Lothians (1839, p. 109). Both writers classified the rock of the sill as ‶greenstone.″ In the Geological Survey Memoir on Central and Western Fife and Kinross (Geikie, Sir Arch., 1900, p. 83) the sill is described as a crystalline dolerite, and its thickness is estimated as at least 450 feet. Dr F. Walker (1923, pp. 366 and 373) regarded the rock as a uniform analcite-dolerite. He commented on its decomposed character at Hallcraig. In the most recent contribution to the study of the sill Day and Stenhouse (1930) noted the occurrence of a picritic phase near the base between high- and low-water mark at Braefoot Bay, and the presence, near the top of the sill at Braefoot Point, of a zone richly charged with xenoliths of quartzite and hornfelsed fine-grained sediments. More detailed investigation has shown that the sill exhibits extraordinary variability in character, both in a vertical direction and when followed along the strike. The present communication is confined to a description of its development between Dalgety Bay and Braefoot Bay, including the promontory of Braefoot Point and that part which lies within the limits